In the design and manufacture of ASIC (application specific integrated circuit) chips, memory chips and microprocessor chips, it is conventional practice to provide the chip designer with a library of conventional circuits from which to generate the design. The circuit designs from which the designer must choose are fixed, and also the rules for interconnecting the circuits by wiring are fixed.
One of the circuit types required by a chip designer is an off-chip driver (OCD) circuit. Usually the chip designer is provided with a plurality of different off-chip driver circuits from which to choose. There is a wide variety of OCD circuits taught in the prior art with each OCD circuit designed to overcome a particular problem. As technologies change and create new problems, new OCD circuits are disclosed to solve the new problems.
The present invention solves the overdrive problem of many of the prior art OCD circuits. OCDs often overdrive an output signal from the ASIC chip and create intolerable levels of ground bounce and noise, causing circuits to fail and corrupting signal integrity. Disclosed is an OCD design which uses precise feedback to the gate of the OCD pull-down transistor to limit the dv/dt and di/dt without using a large resistor as is common for source-follower circuits in the prior art. The disclosed design minimizes the adverse effect on performance. Many of the prior art OCDs are described below.
In U.S. Pat. No. 4,622,482, "Slew Rate Limited Driver Circuit Which Minimizes Crossover Distortion," by Ganger, Ganger teaches a driver circuit which provides an output voltage which is slew rate limited substantially independent of the value of any output load. Ganger's driver is a push-pull driver which uses a pair of transistors of opposite conductivity to drive the output in response to a control signal. Capacitors are utilized to perform slew rate limiting by using a capacitor between each transistor's output and control input. Each transistor is selectively and dynamically biased to insure a substantially linear slew rate. Ganger's driver differs from the present invention in that it is a push-pull driver driving an output between a positive voltage and a negative voltage that provides minimum distortion as the output crosses over from a positive to a negative voltage. The present invention drives the output between a positive voltage and ground and eliminates intolerable levels of ground bounce and noise as the output is driven to ground. In addition, Ganger's driver uses the Miller effect and impedance control of the driver, whereas the present invention does not use the Miller effect.
In U.S. Pat. No. 4,684,824, "Capacitive Load Driver Circuit," by Moberg, Moberg teaches a capacitive load driver which has good response time without cross-conduction. The driver circuit includes a positive drive subcircuit for charging an output load in response to a first input control signal and a negative drive subcircuit for discharging the output load in response to a second control signal. The positive drive subcircuit is used to greatly increase the charging rate during the existence of a predetermined difference between the actual and desired output voltage. The negative drive subcircuit is used to greatly decrease the discharging rate when the discharge exceeds a predetermined value. The output voltage is fed back to detector circuits that make the decisions to boost the positive drive or the negative drive. Moberg's driver differs from the present invention in that it is a push-pull driver driving an output between a positive voltage and ground that uses slew rate feedback and voltage detection circuit, whereas the present invention uses the current output to develop a voltage feedback to the gate electrode of the OCD pull-down device to limit the di/dt and dv/dt. In addition, Moberg's driver is an entirely different circuit that additionally uses positive and negative drive boosters to control slew rate, whereas the present invention is a simpler circuit that does not use Moberg's circuitry containing voltage difference detectors or positive and negative drive boosters.
In U.S. Pat. No. 4,737,667, "Driving Circuitry for a MOSFET Having a Source Load," by Tihanyi, Tihanyi teaches a source-follower circuit for driving a MOS field effect transistor (FET) connected to a load on the source terminal. The source follower is driven by a voltage doubling circuit including two diodes serially connected together. Tihanyi's circuit is a typical prior art driver which uses a large resistance value to slow the turn on of the output pull-down transistor. Tihanyi also uses the typical capacitive feedback between the emitter and gate diode of the driver where the capacitor is also part of the voltage doubling circuitry. Tihanyi's driver differs from the present invention in that it uses a voltage doubling circuit, impedance feedback, and diode control of the input signal, whereas the present invention is a simpler circuit that does not use these features. Tihanyi's capacitor is used as bootstrap drive, whereas the present invention uses its capacitor for level shifting. The present invention, like Tihanyi, uses a source-follower circuit but for the feedback path only.
In U.S. Pat. No. 4,906,867, "Buffer Circuit with Load Sensitive Transition Control," by Petty, Petty teaches a buffer circuit particularly suited for driving the output pad of an integrated circuit. Petty's driver limits the rate of change of current flow in the power supply and ground lines to reduce noise. The circuit features feedback responsive compensation for variations of capacitive load on the pad. The circuit is a composite buffer and output driver which controls the output di/dt through feedback that insures full pull-up of the output stage following transition and controls the turn-on rate of the output transistor to limit the instantaneous current furnished to large capacitive loads. Petty's circuit provides load responsive feedback sensitivity, reduces current surge noise, and ensures concluding DC voltage levels of appropriate magnitudes. The feedback is a capacitor placed between the output and the gate electrode of the driver. Petty differs from the present invention in that it uses the Miller effect and impedance control of the driver, whereas the present invention does not use the Miller effect. Petty uses a buffer circuit, and the present invention does not. The present invention uses a resistor to ground to create and feedback the source-follower potential, whereas Petty does not feedback the source follower potential but instead feeds back the output signal.
In U.S. Pat. No. 5,008,568, "CMOS Output Driver," by Leung et al.,there is taught a driver circuit which uses a transistor configured as a capacitor between the gate and drain of an output pull-down transistor. The purpose is to limit the ground bounce and the rate of change (di/dt) of the current connected through the pull-down transistor during the turn-on of the transistor. Drive for the pull-down transistor is provided in part by a NOR gate, the transistors of which are sized to provide a finite resistance to the pull-down transistor. Additional drive is provided by a transistor connected to function as a resistive pull-up between the gate and the drain of the pull-down transistor. Leung's driver is a push-pull driver which uses a pair of transistors of opposite conductivity to drive the output in response to data and enable input signals. Leung's driver differs from the present invention in that it is a push-pull driver driving an output between a positive voltage and a negative voltage. Leung's driver limits the rate of change of current using a first transistor as capacitor and a second transistor as drive booster. The present invention does not use a push-pull drive, but drives a source-follower output between a positive voltage and ground, while eliminating intolerable levels of ground bounce and noise as the output is driven to ground. The present invention is a simpler circuit that does not use a transistor as a capacitor and does not provide a second transistor and NOR gate to provide the drive from the output circuit. In addition, Leung's driver uses the Miller effect and impedance control of the driver even though the capacitor is implemented using a transistor, whereas the present invention does not use the Miller effect.
In U.S. Pat. No. 5,121,000, "Edge-Rate Feedback CMOS Output Buffer Circuits," by Naghshineh, Naghshineh teaches a CMOS driver circuit that both drives and stores the output value. The CMOS driver provides an output which has a significant reduction in ground bounce. The driver includes an output driver stage, a pull-up pre-driver circuit, a pull-down pre-driver circuit, and feedback means. Naghshineh's driver is a push-pull driver which uses a pair of transistors of opposite conductivity to drive the output in response to both data and enable signals. The feedback means is responsive to the output signal for controlling the rate of rise of the voltage at the gate electrode of the pull-down transistor so as to slow down its turn-on time when the driver output is making a high-to-low transition. A capacitor is utilized on the pull-down transistor to perform slew rate limiting by using a capacitor between the transistor output and the transistors gate electrode. Naghshineh's driver differs from the present invention in that it is a push-pull driver driving an output between a positive voltage and ground that limits the slew rate using a voltage feedback. The present invention uses a simpler and improved circuit that senses the output current rather than the output voltage to establish a feedback voltage.
In U.S. Pat. No. 5,218,239, "Selectable Edge Rate CMOS Output Buffer Circuit," by Boomer, Boomer teaches a driver circuit which controls output signal rise and fall times using a plurality of pull-down pre-driver resistors in parallel. Boomer's driver permits varying the resistor values to select a plurality of different fall times. Boomer's circuit is a typical prior art driver which uses resistance values to slow the turn on of the output pull-down transistor. In symmetrical fashion a plurality of pull-up pre-driver resistors are used in parallel to slow the turn on of the output pull-up transistor. Boomer's driver differs from the present invention in that it is a push-pull driver driving an output between a positive voltage and ground that uses resistor pull-down, whereas the present invention uses feedback to the gate electrode of the OCD pull-down device to limit the di/dt and dv/dt without using a large resistor as a source follower. In addition, Boomer's driver uses the Miller's effect and impedance control of the driver, whereas the present invention does not use a capacitive feedback from the output of the circuit.
In U.S. Pat. No. 5,528,166, "Pulse Controlled Impedance Compensated Output Buffer," by Iikbahar, Iikbahar teaches an improved buffer circuit that uses pulse controlled impedance. The buffer circuit includes two transient drivers which are activated during a portion of the switching time of the output. The two transient drivers are activated for different durations which partially encompass the output transition. The durations act to slow or speed transitions. Iikbahar's driver also includes a feedback circuit which provides impedance compensation by adjusting the total buffer circuit drive. Iikbahar driver differs from the present invention in that it is a push-pull driver driving an output between a positive voltage and ground that uses slew rate feedback and impedance compensation, whereas the present invention uses feedback to the gate electrode of the OCD pull-down device to limit the dv/dt. In addition, Iikbahar's driver is an entirely different circuit that additionally uses two transient drivers to control slew rate, whereas the present invention is a simpler circuit that does not use Iikbahar's circuitry containing two transient drivers.
It is an object of the present invention to provide a clamp circuit for an off-chip driver circuit that actively feeds back the source-follower potential to slow down the OCD and minimize ground bounce and noise that causes circuits to fail and signal integrity to be corrupted.
It is a further object of the present invention to provide a clamp circuit for an off-chip driver circuit that uses feedback to regulate the OCD drive and minimize noise on the output signal.
It is a further object of the present invention to provide a clamp circuit for an off-chip driver circuit that provides precise feedback to the gate of the OCD pull-down transistor to limit dv/dt without using a large resistor as a source follower, hence minimizing the adverse effect on performance.